Implantable device for facet joint replacement

ABSTRACT

Cephalad and caudal vertebral facet joint prostheses and methods of use are provided. The prostheses provide an artificial facet joint structure including an artificial articular configuration unlike the preexisting articular configuration. The radii and material stress values of the prostheses are configured to sustain contact stress. The cephalad prosthesis provides for posterior-anterior adjustment. Both prostheses permit lateral adjustment and adjustment to accomodate interpedicle distance. Further, the prostheses may be customized to provide a pre-defined lordotic angle and a pre-defined pedicle entry angle.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/067,137, filed Feb. 4, 2002, and entitled “FacetArthroplasty Devices and Methods,” which is a continuation-in-part ofU.S. patent application Ser. No. 09/693,272, filed Oct. 20, 2000, andentitled “Facet Arthroplasty Devices and Methods,” which claims thebenefit of Provisional Patent Application Ser. No. 60/160,891, filedOct. 22, 1999, and entitled “Facet Arthroplasty Devices and Methods,”all of which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to prostheses for treating various types ofspinal pathologies, as well as to methods of treating spinalpathologies.

BACKGROUND OF THE INVENTION

I. Vertebral Anatomy

As FIG. 1 shows, the human spinal column 10 is comprised of a series ofthirty-three stacked vertebrae 12 divided into five regions. Thecervical region includes seven vertebrae 12, known as C1-C7. Thethoracic region includes twelve vertebrae 12, known as T1-T12. Thelumbar region contains five vertebrae 12, known as T1-T5. The sacralregion is comprised of five vertebrae 12, known as S1-S5. The coccygealregion contains four vertebrae 12, known as Co1-Co4.

FIG. 2 shows a normal human lumbar vertebra 12. Although the lumbarvertebrae 12 vary somewhat according to location, they share manyfeatures common to most vertebrae 12. Each vertebra 12 includes avertebral body 14. Two short bones, the pedicles 16, extend backwardfrom each side of the vertebral body 14 to form a vertebral arch 18.

At the posterior end of each pedicle 16 the vertebral arch 18 flares outinto broad plates of bone known as the laminae 20. The laminae 20 fusewith each other to form a spinous process 22. The spinuous process 22serves for muscle and ligamentous attachment. A smooth transition fromthe pedicles 16 into the laminae 20 is interrupted by the formation of aseries of processes. Two transverse processes 24 thrust out laterally oneach side from the junction of the pedicle 16 with the lamina 20. Thetransverse processes 24 serve as levers for the attachment of muscles tothe vertebrae 12. Four articular processes, two superior 26 and twoinferior 28, also rise from the junctions of the pedicles 16 and thelaminae 20. The superior articular processes 26 are sharp oval plates ofbone rising upward on each side from the union of the pedicle 16 withthe lamina 20. The inferior processes 28 are oval plates of bone thatjut downward on each side.

The superior and inferior articular processes 26 and 28 each have anatural bony structure known as a facet. The superior articular facet 30faces 30, while the inferior articular facet 31 faces downward. As FIG.3 shows, when adjacent vertebrae 12 are aligned, the facets 30 and 31,capped with a smooth articular cartilage, interlock to form a facetjoint 32, also known as a zygapopysial joint.

The facet joint 32 is composed of a superior half and an inferior half.The superior half is formed by the vertebral level below the joint 32,and the inferior half is formed by the vertebral level above the joint32. For example, in the L4-L5 facet joint, the superior portion of thejoint is formed by bony structure on the L-5 vertebra (e.g., a superiorarticular surface and supporting bone on the L-5 vertebra), and theinferior portion of the joint is formed by bony structure on the L-4vertebra (e.g., an inferior articular surface and supporting bone on theL-4 vertebra).

As also shown in FIG. 3, an intervertebral disc 34 between each pair ofvertebrae 12 permits gliding movement between vertebrae 12. Thus, thestructure and alignment of the vertebrae 12 permit a range of movementof the vertebrae 12 relative to each other.

II. Facet Joint Dysfunction

Back pain, particularly in the “small of the back”, or lumbosacral(L4-S1) region, is a common ailment. In many cases, the pain severelylimits a person's functional ability and quality of life. Such pain canresult from a variety of spinal pathologies.

Through disease or injury, the laminae, spinous process, articularprocesses, or facets of one or more vertebral bodies can become damaged,such that the vertebrae no longer articulate or properly align with eachother. This can result in an undesired anatomy, loss of mobility, andpain or discomfort.

For example, the vertebral facet joints can be damaged by eithertraumatic injury or by various disease processes. These diseaseprocesses include osteoarthritis, ankylosing spondylolysis, anddegenerative spondylolisthesis. The damage to the facet joints oftenresults in pressure on nerves, also called a “pinched” nerve, or nervecompression or impingement. The result is pain, misaligned anatomy, anda corresponding loss of mobility. Pressure on nerves can also occurwithout facet joint pathology, e.g., a herniated disc.

One type of conventional treatment of facet joint pathology is spinalstabilization, also known as intervertebral stabilization.Intervertebral stabilization prevents relative motion between thevertebrae. By preventing movement, pain can be reduced. Stabilizationcan be accomplished by various methods.

One method of stabilization is spinal fusion. Another method ofstabilization is fixation of any number of vertebrae to stabilize andprevent movement of the vertebrae.

Another type of conventional treatment is decompressive laminectomy.This procedure involves excision of the laminae to relieve compressionof nerves.

These traditional treatments are subject to a variety of limitations andvarying success rates. Furthermore, none of the described treatmentsputs the spine in proper alignment or return the spine to a desiredanatomy. In addition, stabilization techniques, by holding the vertebraein a fixed position, permanently limit a person's mobility.

SUMMARY OF THE INVENTION

There is a need for prostheses, systems, and methods that overcome theproblems and disadvantages associated with current strategies anddesigns in various treatments for spine pathologies.

The invention provides prostheses, systems, and methods designed toreplace natural facet joints and/or part of the lamina at virtually allspinal levels including L1-L2, L2-L3, L3-L4, L4-L5, L4-S1, T-11-T12, andT12-L1. The prostheses, systems, and methods can restore a desiredanatomy to a spine and give back to an individual a desired range ofmobility. The prostheses, systems, and methods also can lessen oralleviate spinal pain by relieving the source nerve compression orimpingement.

For the sake of description, the prostheses that embody features of theinvention will be called either “cephalad” or “caudal” with relation tothe portion of a given natural facet joint they replace. As previouslydescribed, a given natural facet joint has a superior half and aninferior half. In anatomical terms, the superior half of the joint isformed by the vertebral level below the joint (which can thus be calledthe caudal portion of the facet joint, i.e., because it is near thefeet). The inferior half of the joint is formed by the vertebral levelabove the joint (which can thus be called the cephalad portion of thefacet joint, i.e., because it is near the head). Thus, a prosthesisthat, in use, replaces the caudal portion of a facet joint (i.e., thesuperior half) will be called a “caudal” prosthesis. Likewise, aprosthesis that, in use, replaces the cephalad portion of a facet joint(i.e., the inferior half) will be called a “cephalad” prosthesis.

One aspect of the invention provides a facet joint prosthesis toreplace, on a vertebral body, a caudal portion of a natural facet joint(e.g., a superior articular surface and supporting bone structure on thevertebral body). The caudal prosthesis comprises a component sized to befixed to the vertebral body, e.g., on or near a pedicle. The caudalprosthesis includes an artificial facet joint structure adapted toreplace a caudal portion of the natural facet joint after its removalfrom the vertebral body. The removal of a caudal portion of the naturalfacet joint and its total replacement by the artificial facet jointstructure of the caudal prosthesis frees the orientation of theprosthesis from anatomic constraints imposed by a preexisting articularconfiguration of the caudal portion of the natural facet joint.Furthermore, the artificial facet joint structure of the caudalprosthesis can comprise an artificial articular configuration that isunlike the preexisting articular configuration, so that a desiredarticulation or bony anatomy can be restored.

This aspect of the invention also provides a method of replacing, on avertebral body, a caudal portion of a natural facet joint. The methodremoves a caudal portion of the natural facet joint from the vertebralbody, and, in its place, fixes a component to the vertebral body thatincludes an artificial facet joint structure adapted to replace theremoved caudal portion of the natural facet joint. The artificial facetjoint structure can include an artificial articular configuration unlikethe preexisting articular configuration of the removed caudal portion ofthe natural facet joint.

Another aspect of the invention provides a facet joint prosthesis toreplace, on a vertebral body, a cephalad portion of a natural facetjoint (e.g., an inferior articular surface and supporting bone structureon the vertebral body). The cephalad prosthesis comprises a componentsized to be fixed to the vertebral body, e.g., on or near a pedicle, oron or near a lamina, or on or near a spinous process, or combinationsthereof. The cephalad prosthesis includes an artificial facet jointstructure adapted to replace a cephalad portion of the natural facetjoint after its removal from the vertebral body. As with the removal andtotal replacement of a caudal portion of the natural facet joint, theremoval of a cephalad portion of the natural facet joint and its totalreplacement by the artificial facet joint structure of the cephaladprosthesis makes possible the orientation of the prosthesis free fromanatomic constraints imposed by a preexisting articular configuration ofthe cephalad portion of the natural facet joint. Furthermore, like thecaudal prosthesis, the artificial facet joint structure of the cephaladprosthesis can comprises an artificial articular configuration that isunlike the preexisting articular configuration of the natural facetsurface (which is removed), so that a desired articulation or bonyanatomy can be totally restored.

This aspect of the invention also provides a method of replacing, on avertebral body, a cephalad portion of a natural facet joint. The methodremoves a cephalad portion of the natural facet joint from the vertebralbody, and, in its place, fixes a component to the vertebral body thatincludes an artificial facet joint structure adapted to replace theremoved cephalad portion of the natural facet joint. The artificialfacet joint structure can include an artificial articular configurationunlike the preexisting articular configuration of the removed cephaladportion of the natural facet joint.

Another aspect of the invention provides a prosthesis assembly andrelated method for replacing a natural facet joint between adjoiningfirst and second vertebral bodies. The assembly and method utilize afirst component sized to be fixed to the first vertebral body, which issuperior to the second vertebral body. The first component includes afirst artificial facet joint structure adapted to replace a cephaladportion of the natural facet joint on the first vertebral body afterremoval of the cephalad portion of the natural facet joint from thefirst vertebral body. The assembly and method also comprise a secondcomponent sized to be fixed to the second vertebral body. The secondcomponent includes a second artificial facet joint structure adapted toreplace the caudad portion of the natural facet joint of the secondvertebral body after removal of the caudad portion of the natural facetjoint from the second vertebral body. Together, the first and secondartificial facet joint structures comprise an artificial facet joint.The removal of both cephalad and caudal portions of a natural facetjoint and their total replacement by the artificial facet jointstructures of the first and second components allows the artificialfacet joint to be installed without anatomic constraints imposed by apreexisting articular configuration of the natural facet joint.Furthermore, the artificial facet joint structures of either the firstor second components, or both, can comprise create an artificialarticular configuration for the artificial facet joint that is unlikethe preexisting articular configuration of the removed natural facetjoint, so that a desired articulation or bony anatomy can be completelyrestored.

Various other aspects of the invention provide caphalad and/or caudalprostheses that readily adapt to or physically change the specificanatomy of an individual. For example, a cephalad prosthesis can becapable of being adjusted in either an anterior or posterior directionrelative to a vertebra. As another example, a cephalad prosthesis and/ora caudal prosthesis can provide for lateral (left and right) adjustment,to accommodate or create variances in the distance between the right andleft pedicles of a single vertebra. Furthermore, a cephalad prosthesisand/or a caudal prosthesis can provide vertical (up and down)adjustment, to accommodate or create variations in interpedicle distancebetween adjacent vertebra. Or, as another example, a cephalad prosthesisand a caudal prosthesis can together create a desired lordotic anglebetween adjacent vertebral bodies, or create a pre-defined pedicle entryangle for mounting each prosthesis on a given vertebral body. And, asyet another example, the configuration of articulating artificial facetjoint structures on cooperating caphalad and caudal prostheses can bematched, taking into account the material(s) from which they are made,to minimize contact stress.

Another aspect of the invention provides an intermediate prosthesisthat, together with the cephalad and caudal prostheses, makes possiblemultiple-level facet joint replacement.

Other features and advantages of the inventions are set forth in thefollowing Description and Drawings, as well as in the appended claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lateral elevation view of a normal human spinal column;

FIG. 2 is a superior view of a normal human lumbar vertebra;

FIG. 3 is a lateral elevation view of a vertebral lumbar facet joint;

FIG. 4 is a perspective view of a cephalad prosthesis for replacing theinferior half of a natural facet joint;

FIG. 5 is a perspective view of a caudal prosthesis for replacing thesuperior half of a natural facet joint;

FIG. 6 is a perspective view illustrating the cephalad prosthesis shownin FIG. 4 in articulation with the caudal prosthesis shown in FIG. 5;

FIG. 7 is an exploded perspective view of an alternative embodiment of acaudal prosthesis for replacing the superior half of a natural facetjoint, the prosthesis having a multiple-piece construction;

FIG. 8 is an assembled perspective view of the caudal prosthesis shownin FIG. 7;

FIG. 9 is a rear view of the caudal prosthesis shown in FIG. 8;

FIG. 10 is a side elevation view of the caudal prosthesis shown in FIG.8, illustrating, in phantom lines, anterior-posterior adjustment of thearm components;

FIG. 11 is a top view of the caudal prosthesis shown in FIG. 8;

FIG. 12 is a front view of a cephalad prosthesis for replacing theinferior half of a natural facet joint, illustratinghorizontally-elongated openings that accommodate lateral adjustment ofthe prosthesis;

FIG. 13 is a front view of a caudal prosthesis for replacing thesuperior half of a natural facet joint, illustratinghorizontally-elongated openings that accommodate lateral adjustment ofthe prosthesis;

FIG. 14 is a front view of a cephalad prosthesis for replacing theinferior half of a natural facet joint, illustratingvertically-elongated openings that accommodate adjustment of theprosthesis to accommodate varying interpedicle distances;

FIG. 15 is a front view of caudal prosthesis for replacing the superiorhalf of a natural facet joint, illustrating vertically-elongatedopenings that accommodate adjustment of the prosthesis to accommodatevarying interpedicle distances;

FIG. 16 is a side view of the cephalad prosthesis shown in FIG. 4 inarticulation with the caudal prosthesis shown in FIG. 5, illustratingthe orientation of the fixation openings relative to a superior-inferioraxis to provide a pre-defined lordotic angle;

FIG. 17 is a top view of the cephalad prosthesis shown in FIG. 4 inarticulation with the caudal prosthesis shown in FIG. 5, illustratingthe orientation of the fixation openings relative to a lateral axis toprovide a pre-defined pedicle entry angle;

FIG. 18 is an exploded perspective view of a multiple-level prosthesissystem comprising a cephalad prosthesis for replacing the inferiorhalves of natural facet joints on a first superior vertebral body, acaudal prosthesis for replacing the superior halves of natural facetjoints on a second inferior vertebral body, an intermediate prosthesisfor replacing both inferior and superior halves of natural facet jointson a third vertebral body between the first and second vertebral bodies;

FIG. 19 is an assembled perspective view of the multiple-levelprosthesis system shown in FIG. 18;

FIG. 20 is a perspective view of a representative embodiment of acephalad prosthesis for replacing the inferior half of a natural facetjoint;

FIG. 21 is a front view of the cephalad prosthesis shown in FIG. 20;

FIG. 22 is a top view of the cephalad prosthesis shown in FIG. 20;

FIG. 23 is a side view of the cephalad prosthesis shown in FIG. 20;

FIG. 24 is a rear view of the cephalad prosthesis shown in FIG. 20;

FIG. 25 is a perspective view of a representative embodiment of a caudalprosthesis for replacing the superior half of a natural facet joint;

FIG. 26 is a rear view of the caudal prosthesis shown in FIG. 25;

FIG. 27 is a front view of the caudal prosthesis shown in FIG. 25;

FIG. 28 is a side view of the caudal prosthesis shown in FIG. 25;

FIG. 29 is a top view of the caudal prosthesis shown in FIG. 25;

FIG. 30 is a perspective view of the cephalad prosthesis shown in FIG.20 in articulation with the caudal prosthesis shown in FIG. 25;

FIG. 31 is a front view of the articulated prostheses shown in FIG. 30;

FIG. 32 is a side view of the articulated prostheses shown in FIG. 30;

FIG. 33 is a rear view of the articulated prostheses shown in FIG. 30;

FIG. 34 is a top view of the articulated prostheses shown in FIG. 30;

FIG. 35 is a posterior perspective view of the natural left and rightfacet joints between two lumbar vertebrae;

FIG. 36 is a posterior perspective view of the lumbar vertebrae shown inFIG. 35, showing the surgical removal of the natural inferior processesand related bony structure of the superior vertebra and the surgicalremoval of the natural superior processes and related bony structure ofthe inferior vertebra;

FIG. 37 is a posterior perspective view of the lumbar vertebrae shown inFIGS. 35 and 36, after removal of the inferior and superior halves ofthe natural facet joints, illustrating the fixation of a caudalprosthesis onto the inferior vertebra for replacing the superior halvesof the natural facet joints that have been removed; and

FIG. 38 is a posterior perspective view of the lumbar vertebrae shown inFIG. 37, after removal of the inferior and superior halves of thenatural facet joints and the fixation of the caudal prosthesis onto theinferior vertebra, illustrating the fixation of a cephalad prosthesis onthe superior vertebra for replacing the inferior halves of the naturalfacet joints that have been removed, the caudal and cephalad prosthesesarticulating to provide an artifical articulation that can be unlike thenatural articulation of the removed natural facet joints.

The invention may be embodied in several forms without departing fromits spirit or essential characteristics. The scope of the invention isdefined in the appended claims, rather than in the specific descriptionpreceding them. All embodiments that fall within the meaning and rangeof equivalency of the claims are therefore intended to be embraced bythe claims.

DETAILED DESCRIPTION

Although the disclosure hereof is detailed and exact to enable thoseskilled in the art to practice the invention, the physical embodimentsherein disclosed merely exemplify the invention that may be embodied inother specific structure. While the preferred embodiment has beendescribed, the details may be changed without departing from theinvention, which is defined by the claims.

I. Vertebral Prostheses

FIGS. 4 to 6 illustrate various prostheses for replacing inferior and/orsuperior portions of natural facet joints. The prostheses are desirablyfixed to vertebral bodies following the surgical removal of therespective natural facet joint portions from the vertebral bodies.

FIG. 4 shows a cephalad prosthesis 36 for replacement of the naturalinferior half of a facet joint following removal of the natural inferiorhalf of the facet joint. FIG. 5 shows a caudal prosthesis 38 forreplacement of the natural superior half of a facet joint followingremoval of the natural superior half of the facet joint.

Either prosthesis 36 or 38 can be used by itself. The prostheses 36 and38 each enables bilateral facet joint replacement (both left and rightsides of a given vertebral body), or unilateral facet joint replacement(one side of a given vertebral body).

As shown in FIG. 6, the prostheses are desirably used in articulatedassociation between a given pair of vertebral bodies. As FIG. 6 shows,the caudal and cephalad prostheses 36 and 38 form an articulated systemthat permits total (superior and inferior) facet joint replacement ofone or more natural facet joints 32. The system can provide a successionof entirely artificial facet joint structures between two vertebralbodies or along a length of the spinal column 10.

As shown in FIG. 6, the caudal and cephalad prostheses 36 and 38cooperate in combination to provide an artificial articularconfiguration. Since the inferior and superior halves of the naturalfacet joint are removed, the artificial articular configuration need notbe constrained by, and can be unlike, the preexisting articulation ofthe natural facet joint, prior to the removal of the inferior andsuperior halves of the natural facet joint.

A. The Cephalad Prosthesis

The prosthesis 36 shown in FIG. 4 is designated “cephalad” because itprovides one or more artificial facet joint structures 40 for theinferior half of a natural facet joint 32. The prosthesis 36 allows forthe removal of injured, diseased and/or deteriorating natural inferiorarticular surfaces 28 and supporting bony structure on the vertebra 12above the facet joint 32. The artificial structures 40 serve to replacethe natural inferior processes 28 and supporting bone of the vertebralbody, which have been desirably removed prior to mounting the prosthesis36 on the vertebral body, as will be described in greater detail later.

The artificial facet joint structures 40 articulate with the superiorhalf of the facet joint 32. The superior half can comprise the naturalsuperior portions of the facet joint 32 (i.e., the natural superiorarticular surfaces 26 and supporting bony structure on the vertebralbody below the facet joint 32). Desirably, however, the superior halfcomprises an artificial facet joint structure 54 formed by a caudaljoint replacement prosthesis 38, shown, e.g., in FIG. 6.

The cephalad prosthesis 36 is sized to extend across the laminae 20 of avertebral body. In the illustrated embodiment, the caphalad prosthesiscomprises a chimney 42 and left and right arm components 44.

The chimney 42 is configured to receive the spinous process 22 of thevertebral body In this manner, the chimney 42 serves to support andstabilize the prosthesis 36. The chimney 42 desirably includes a laminahook 46 (best shown in FIG. 7) that rests under the laminae 20 of thevertebral body to further support the prosthesis 36. If desired, thespinous process 22 may be fixed within the chimney 42 with atrans-spinous process screw 48. In some instances, when a significantportion of the laminae 20 is removed, it may be desirable to omit thechimney 42 entirely.

The chimney 42 carries right and left arms 44 in association with thechimney 42. Each arm 44 carries an artificial facet joint structure 40.

As seen in FIGS. 4 and 6, the arms 44 comprise an inferior-to-superiordiverging geometry, with a greater lateral width at the superior endthan the inferior end, facilitate mounting to the pedicles of avertebral body. However, other configurations can be used.

Each arm 44 additionally carries at least one opening 50 configured toreceive a fixation element 52 for fixing the prosthesis 36 to thevertebral body. It should be understood that the number and location ofopenings 50 and fixation elements 52 could vary.

In FIG. 4, two openings 40 (right and left) serve to receive twofixation elements 52 (right and left). In the illustrated embodiment,the fixation elements 52 take the form of pedicle screws or nails. Theright and left fixation elements 52 are adapted to extend into the rightand left pedicles 16 respectively of the vertebral body and serve toanchor the prosthesis 36 in place.

As shown in FIG. 4, the cephalad prosthesis 36 can be of unitaryconstruction, in which the chimney 42 and arms 44 are a single piece.Alternatively, as shown in FIG. 7, the prosthesis 36 can be ofmultiple-piece construction, in which the arms 44 are configured to beselectively detached from the chimney 42, as will be discussed ingreater detail later with respect to FIG. 7.

The cephalad prosthesis 36 may be formed of a material commonly used inthe prosthetic arts including, but not limited to, polyethylene, rubber,titanium, chrome cobalt, surgical steel, bony in-growth sintering,sintered glass, artificial bone, ceramics, or a combination thereof.

B. The Caudal Prosthesis

The prosthesis 36 shown in FIG. 5 is designated “caudal” because itcreates one or more artificial facet joint structures 54 for thesuperior half of a natural facet joint. The caudal prosthesis 38 allowsfor the removal of injured, diseased and/or deteriorating naturalsuperior articular surfaces 32 and supporting bony structure on thevertebral body below the facet joint 32. The artificial structures 54serve to replace the natural superior processes 26 and supporting boneof the vertebral body, which have been desirably removed prior tomounting the prosthesis 38 on the vertebral body. This aspect will bedescribed in greater detail later.

In use, the artificial facet joint structure 54 articulates with theinferior half of the facet joint 32. The inferior half can comprise thenatural inferior portions of the facet joint 32 (i.e., the naturalinferior articular surfaces and supporting bony structure on thevertebral body above the facet joint 32). Desirably, however, theinferior half comprises an artificial facet joint structure 40 formed bya cephalad joint replacement prosthesis 36, as FIG. 6 shows.

In the illustrated embodiment, the caudal prosthesis 38 is a bar-likemember sized to extend across the laminae 20 of the vertebral body.While FIG. 4 illustrates a unitary construction, the prosthesis 38 canbe constructed multiple-parts that are joined together for use.

Like the cephalad prosthesis 36, the caudal prosthesis 38 carries atleast one opening 56 configured to receive a fixation element 58 forfixing the prosthesis 38 to the vertebral body and at least oneartificial facet joint structure element 54. It is to be understood thatthe number and location of openings and fixation elements can vary.

In FIG. 5, two openings 56 (right and left) serve to receive twofixation elements 58 (right and left). In the illustrated embodiment,the fixation elements 58 take the form of pedicle screws or nails. Theright and left fixation elements 58 are adapted to extend into the rightand left pedicles respectively of the vertebral body and serve to anchorthe prosthesis 38 in place.

The caudal prosthesis 38 may be formed of a material commonly used inthe prosthetic arts including, but not limited to, polyethylene, rubber,titanium, chrome cobalt, surgical steel, bony in-growth sintering,sintered glass, artificial bone, ceramics, or a combination thereof.

II. Additional Features of the Prostheses

Either or both of the cephalad and caudal prostheses 36 and 38 canincorporate a variety of additional features, which adapt the prosthesis36 or 38 to the specific anatomy encountered or desired. These adaptivefeatures further enhance the restoration of a desired anatomy and/or thealleviated of pain, as will be described in greater detail later. Aswill become apparent to one skilled in the art, any of the followingfeatures can be used alone or in combination with any other feature orfeatures, to “customize” a prosthesis 36 or 38 to a given vertebrallocation and a specific individual.

A. Posterior-Anterior Adjustment

As shown in FIGS. 7 to 11, the cephalad prosthesis 36 can comprise amultiple-piece construction. The multiple-piece construction permitsposterior and anterior (i.e., “front and back”) adjustment of theprosthesis 36 relative to the vertebral body. Either symmetric orasymmetric posterior-anterior mounting arrangements are thereby enabled.

In the embodiment shown in FIGS. 7 to 11, the prosthesis 36 is athree-piece assembly comprising a center member 42 (which can comprisethe chimney already discussed) and right and left arm components 44.Each arm 44 is a separate piece that is selectively detachable from thecenter member 42. The arms 44 can be coupled to the center member 42 bya variety of means, including, but not limited to, a slotted jointbetween the chimney 42 and the respective arm 44, a screw attachment, ahook attachment, or a snap-fit engagement.

A slotted joint or the like allows for relative sliding movement betweenthe respective arm and center member. As best seen in FIG. 10, thisarrangement permits independent posterior and anterior adjustment(represented by phantom lines) of the right and left arms 44. Theadjustment allows the prosthesis 36 to accommodate the asymmetricanterior-posterior anatomy of a particular vertebral body.

In an alternative construction (not shown), the center member 42 cancomprise one piece and the arms 44 are integrally attached to form asecond, discrete piece that is selectively attachable and detachablefrom the member 42. This arrangement would similarly permit posteriorand anterior adjustment of the arms 44, but would not permit independentadjustment of the right and left arms 44.

The orientation of the prosthesis 36 is thus not dictated by the naturalposterior-anterior anatomy encountered. Instead, the prostheses 36 canbe fixed in position between two vertebral bodies in an orientation thatcorresponds to existing natural anatomy or that establishes a desiredposterior-anterior anatomy unlike the natural anatomy.

B. Lateral Adjustment

As seen in FIGS. 12 and 13, any or all of the openings 50 and 56 of thecephalad prosthesis 36 and caudal prosthesis 38 can behorizontally-elongated, i.e., oriented transverse the superior-inferioraxis of the prosthesis 36 or 38. The horizontal configuration permitslateral adjustment (i.e., “right to left”) of the prosthesis 36 or 38relative to a vertebral body (see also FIGS. 1-3). Thus, thisarrangement allows for variance in distance between the right and leftpedicles 16 of a single vertebra 12.

The horizontal configuration allows the fixation elements 52 and 58 onopposite lateral sides of the prostheses 36 and 38 to be placed anywherebetween an “A” position (illustrated by solid lines in FIGS. 12 and 13)and a “B” position (illustrated by phantom lines in FIGS. 12 and 13).

The lateral orientation of the artificial facet joint structures of theprostheses 36 or 38 is thus not dictated by the natural lateral anatomy(i.e., intrapedicular distance) encountered. Instead, the position ofthe artificial facet joint structures of the prostheses 36 and 38 can bechanged relative to the position of the pedicles; either medial to orlateral to the pedicles (establishing a desired lateral anatomy unlikethe natural anatomy), or in an orientation that corresponds to anexisting natural anatomy.

C. Adjustment of Interpedicle Distance

Referring now to FIGS. 14 and 15, any or all of the openings 50 and 56of either or both of the cephalad and caudal prostheses 36 and 38 can bevertically-elongated along the superior-inferior axis of the prosthesis36 or 38. The vertical arrangement permits superior and inferior (i.e.,“up and down”) adjustment of the prosthesis 36 or 38 relative to avertebral body (see also FIGS. 1-3).

The vertical configuration allows the fixation elements 52 and 58 to beplaced anywhere from an “A” position (illustrated by solid lines inFIGS. 14 and 15) and a “B” position (illustrated by phantom lines inFIGS. 14 and 15). This arrangement permits the distances between thepedicles 16 of adjacent vertebral bodies to be varied. It thereforeaccommodates asymmetric pedicle 16 orientation (i.e., lateral alignmentof the pedicle 16) while maintaining vertical alignment of theprosthesis 36 or 38.

The orientation of the prostheses 36 and 38 is not dictated by thenatural interpedicular distances encountered. Instead, the prostheses 36and 38 can be fixed in position between two vertebral bodies in anorientation that corresponds with the existing natural anatomy or thatestablishes a desired interpedicular distance unlike the naturalpreexisting interpedicle distance. The prostheses 36 and 38 therebyserve to create a desired interpedicular distance for the vertebralbodies consistent with a desired anatomy.

D. Lordotic Angle Adjustment

As shown in FIG. 1, the cervical and lumbar regions of the spinal column10 normally have an anteriorly convex curvature, known as lordosis. Thecurvature defines a lordotic angle between adjacent vertebral bodies,which is defined by the superior/inferior orientation of the end platesof adjacent vertebral bodies. The lardotic angle varies between adjacentvertebral bodies along the spine. A desired anatomy results bymaintaining desired lordotic angles along the spine, which assuresdesired ligament distention and posture.

In the illustrated embodiment (see FIG. 16), the openings 50 and 56 ofadjacent cephalad and caudal prostheses 36 and 38 are mutually orientedin non-parallel planes along the inferior-superior axis. Thenon-parallel orientation of the planes defines between the fixationelements 52 and 58, when supported by the openings 50 and 56, an anglethat results a desired lordotic angle. The mutual orientation and theresulting angle defined depends upon the intended location of theprostheses 36 and 38 along the spinal column 10.

The defined angle is designated angle “L” in FIG. 16. In FIG. 16, theangle L is defined by orienting the plane of the opening 50 of thecephalad prosthesis 36 generally parallel to the inferior-superior axis,while tilting the plane of the opening 56 of the caudal prosthesis 38generally downward at an acute inferior angle relative to theinferior-superior axis. The resulting defined angle L between thefixation elements 52 and 58 in FIG. 16 (about 15°) achieves a desiredlordotic angle for the L4-L5 level.

The orientation of the prostheses 36 and 38 is not dictated bypreexisting natural lordotic angle between two vertebral bodies.Instead, the prostheses 36 and 38 can be fixed in position between twovertebral bodies in an orientation that establishes a desired lordoticangle unlike the natural preexisting angle. The prostheses 36 and 38thereby serve to create a desired lordotic angle for the vertebralbodies consistent with a desired anatomy.

Changes in the thickness of and/or orientation of the artificial facetjoint structures on either or both prostheses 36 and/or 38 can alsoaffect a desired lardotic angle between adjacent vertebral bodies.

E. Adjustment of Pedicle Entry Angle

As shown in FIG. 2, the pedicles 16 extend from a vertebral body at anangle. With reference to FIG. 17, to fix the fixation elements 50 and 58securely within the pedicle 16, it is desirable for the fixationelements 52 and 58 to enter the pedicle 16 at an angle approximating thenatural angle of the pedicle 16, e.g., about 15° at the L4 and L5 level.

To achieve the desired angle, the openings 50 and 56 of adjacentcephalad and caudal prostheses 36 and 38 are mutually tilted inwardly todefine between a lateral axis and the fixation elements 52 and 58, whensupported by the openings 50 and 56, an angle that approximates adesired pedicle entry angle P1/P2. In the illustrated embodiment (FIG.17), the right fixation element 52 or 58 extends at a first angle(designated “P1” in FIG. 17) and the left fixation element 52 or 58extends at a second angle (designated “P2” in FIG. 17) In theillustrated embodiment, P1 is generally the same as P2. However, theangles P1 and P2 can differ.

While the illustrated embodiment shows cephalad and caudal prostheses 36and 38 having the same pedicle entry angles (i.e., P1 and P2 are thesame for the cephalad and caudal prostheses 36 and 38), it is to beunderstood that the cephalad and caudal prostheses 36 and 38 can beformed to have different pedicle entry angles.

F. Multiple-Level Replacement

As shown in FIGS. 18 and 19, the cephalad and caudal prostheses 36 and38 can be coupled by an intermediate prosthesis 60, permittingmultiple-level facet joint replacement. The cephalad prosthesis 36 isplaced on a most superior vertebral body, e.g., L3. The intermediateprosthesis 60 is placed on the next adjacent inferior vertebral body,e.g., L4. The caudal prosthesis 38 is placed on the next adjacentinferior vertebral body, e.g., L5.

The intermediate prosthesis 60 is similar to the cephalad prosthesis 36previously described, having a chimney 42 and two openings (right andleft) 50 that receive fixation elements 52. Right and left arms 44provide a first and second pairs of artificial facet joint structures 64and 66. The first pair 64 is configured to replace the superiorprocesses 26 and related bony structure of the middle vertebral body,and to articulate with the artificial facet joint surfaces 40 of thecephalad prosthesis 36. The second pair 66 is configured to replace theinferior processes 28 and related bony structure of the middle vertebralbody, and to articulate with the artificial facet joint surfaces 54 ofthe caudal prosthesis 38.

G. Ability to Sustain Contact Stress

In the prostheses 36 and 38, each artificial facet joint structure 40/54creates a bearing surface having a configuration that facilitatesarticulation with the bearing surface of another artificial facet jointstructure. The particular geometry for the bearing surface configurationfor a given artificial facet joint structure 40/54 can vary. It can, forexample, be concave, convex, or flat. It may also include a hybrid ofcurved and flat bearing surface designs, i.e., Miniscal, hinge, etc.

The radii of two articulating bearing surface configurations aredesirably selected and matched, taking into account the material fromwhich the surfaces are formed, to minimize contact stress duringarticulation.

For example, in the embodiment illustrated in FIG. 16, the cephaladprosthesis 36 includes artificial facet structures 40 employinggenerally concave surfaces 68, forming socket-like artificial facetjoint structures. In this arrangement, the caudal prosthesis 38 includesartificial facet structures 54 employing generally complementary convexsurfaces 70, forming hemisphere-like artificial facet joint structuresthat articulate with the socket-like artificial facet joint structures.It should be appreciated that the articulating surfaces can be reversed,with the artificial facet structures 40 of the cephalad prosthesis 36employing generally hemispherical-like surfaces, and the artificialfacet structures 54 of the caudal prosthesis 38 employing generallysocket-like surfaces.

Alternatively, a Miniscal bearing design could be employed, utilizing aconformal curved surface as one artificial facet joint structure, withthe bearing side of the opposed artificial facet joint structure havingan essentially flat surface. A hemiarthroplasty design could alsoalternatively be employed, in which one surface of the opposing surfacesdoes not incorporate the use of an artificial facet joint structure.

In another arrangement, one surface of an artificial facet jointstructure can have bearing articulation on oth sides of the componentand have opposing articulation with a receiving artificial facet jointstructure with having opposing mating bearing surfaces.

A variety of materials are suitable for the artificial facet jointstructures. Ceramic or ceramic in opposition with a chrome alloy can beused. Suitable stainless steel, including 3161, or titanium alloys, withor without the use of surface hardening and overlay, or hard surfacecoatings, including zirconia and alumina, can also be employed. Themetal surfaces can be made from cast, wrought, hot-forged, orpowder-metal consolidated sintered materials. Any of these metals orcombination of metals and ceramics can be used in articulation with eachother. Biocompatible polymers, e.g., polyethylene, can also be used inarticulation with the metals, ceramic, and surface-hardened metals justdescribed. Ultra High Molecular Weight Polyethylene can further begamma-irradiated, as-molded or as-machined.

The radii of articulating artificial facet joint structures aredesirably closely matched to provide contact stress values less than agiven threshold value. The desired contact stress value changes with thematerial employed.

For example, the contact stress value for metal-to-metal bearingcombinations is desirably less than about 25,000 psi, and preferablyless than 12,000 psi. For polymer surfaces bearing against a metal,ceramic, or surface-hardened metal counter bearing surface, the contactstress value is desirably less than 10,000 psi, and preferably less than5,000 psi.

For a given material to achieve a desired contact stress value less thanthe threshold value, the appropriate radii must be chosen. Thus, theradii chosen will change as material changes.

III. Representative Embodiments

A. Cooperating Caphalad and Caudal Prostheses

FIGS. 20 to 24 show a representative embodiment of a cephalad prosthesis36 that embody features previously described.

The prosthesis 36 comprises right and left arm components 44 joined to achimney 42 in a single-piece, unitary construction. Each arm 44 includesan artificial facet joint structure 40 (right and left). As best seen inFIG. 23, each artificial facet joint structure 40 comprises a concavesurface 70, forming a socket-like bearing surface.

In the illustrated embodiment (see FIG. 24), the fixation openings 50are vertically-elongated, thereby permitting adjustment of theprosthesis 36 to create a desired interpedicle distance. As best seen inFIG. 22, both the right and left openings 50 are also oriented inward,to provide a pre-defined pedicle entry angle for the fixation elements52.

FIGS. 25 to 29 show a representative embodiment of a caudal prosthesis38 that embody features previously described and that is intended to beused in articulation with the cephalad prosthesis 36 shown in FIGS. 22to 24. FIGS. 31 to 34 show the caudal prosthesis 38 in articulation withthe cephalad prosthesis 36.

The prosthesis 38 includes a pair of artificial facet joint structures54 (right and left). Each artificial facet structure element 54 includesa convex surface 68, forming hemaspherical-like bearing surface. Thesurfaces 68 are intended, in use, to articulate with the socket-shapedbearing surfaces 40 on the cephalad prosthesis 36 (see FIG. 32).

In the illustrated embodiment, the openings 56 are vertically-elongated,thereby permitting adjustment of the prosthesis 38 to create a desiredinterpedicle distance. The vertical openings 50 and 56 on the prostheses36 and 38 permit each prosthesis 36 and 38 to be independently adjustedto create a desired interpedicle distance.

As FIG. 28 best shows, the openings 56 are also oriented inward anddownward. The inward orientation establishes a pre-defined pedicle entryangle for the fixation elements 58. The downward orientation of thefixation element 58 for the caudal prosthesis 38, in combination withthe different, non-parallel orientation of the fixation element 52 forthe cephalad prosthesis 36, establishes a desired lordotic angle.

B. Total Facet Replacement Using the Cephalad and Caudal Prostheses

With reference now principally to FIGS. 35 to 38, both the superior andinferior portions of the natural facet joint 32 are removed and replacedby the cephalad prosthesis 36 and the caudal prosthesis 38. Moreparticularly, the inferior lamina 20 and the inferior portion of thenatural facet joint 32 (e.g., the articulated inferior processes 28 andits supporting bone of the vertebral body 14 above the facet joint) isremoved. The lamina may additionally be cut for a wide decompressivelaminectomy along a decompressive superior-to-inferior resection line onone or both sides of the vertebral body. The removed natural anatomy isreplaced with the cephalad prosthesis 36. The superior portion of thenatural facet joint 32 (e.g., the articulated superior process 26 andits supporting bone of the targeted vertebral body 14) is also removed.Desirably, the mamillary process, the accessory process, a portion ofthe transverse process, and a portion of the pedicle is removed by beingrongeured or reamed. The removed natural anatomy is replaced with thecaudal prosthesis 38.

In one embodiment, a surgical procedure exposes the spinous process 22,lamina 20, and facet joints 32 at a desired level of the spine 10 usingany method common to those of skill in the medical arts. FIG. 35 showsthe exposed spinous process 22, lamina 20, and facet joint 32 of theL4-L5 joint.

The inferior portion of the facet joint 32 is cut at or near a selectedresection line. Most of the lamina 20 is desirably preserved, as is thefacet joint capsule, which may be opened and folded back. The facetjoint capsule may be cut perpendicular to its direction. The naturalinferior portion of the facet joint 32 may then be retracted from thesuperior portion. Once the inferior and superior portions of the facetjoint are separated, the cut inferior bone, e.g., the inferior articularprocess 28 and its supporting bone, of the upper joint (e.g., the cutinferior portion of the L4 vertebra in the L4-L5 joint) may be removed,as depicted by phantom lines In FIG. 36. Alternatively, it may bepossible to remove the cut inferior bone while simultaneously separatingthe facet joint 32.

Prominent bone of the superior portion of the natural facet joint, e.g.,the superior articular process 26 and its supporting bone, may be alsoremoved, as depicted by phantom lines in FIG. 36, using any means commonin the field. The superior portion of the natural facet joint 32 mayalso be trimmed to decompress the adjacent nerve root. A reamer or anyother instrument that is useful for grinding or scraping-bone, may beused to ream the superior portion of the facet joint 32 into the pedicle16, to reach the geometry shown in FIG. 36, which is suitable forreceiving the caudal prosthesis 38.

With reference to FIG. 37, the caudal prosthesis 38 as described abovecan be directly screwed or tapped into the vertebral body 14 usingpedicle screws or other fixation elements 58.

With reference now to FIG. 38, the cephalad prosthesis 36, as describedabove, can be installed over the spinous process 22 and over the lamina20, either before or after placement of the caudal prosthesis 38. Thecephalad prosthesis 36 can be directly screwed or tapped using pediclescrews or other fixation elements 52 to the lamina 20 and to eachpedicle 16. The cephalad prosthesis 36 can also be further attached tothe spinous process 22 with a trans-spinous-process screw 48 to provideadditional stability, as also previously described.

As shown in FIG. 38, articulation between the artificial facet jointstructures 40/54 of the cephalad prosthesis and the caudal prosthesis 36and 38 is established, to provide an artificial articular configurationin place of the preexisting natural articular configuration.

Because both the superior and inferior portions of the natural facetjoint 32 and surrounding bone structures have been removed, theartificial facet joint structures 40/54 of the cephalad prosthesis 36and the caudal prosthesis 38 can be installed in desired positions andorientations, free of anatomic constraints imposed by the preexistingarticular configuration of either the inferior or superior portions ofthe natural facet joint 32. Furthermore, the artificial facet jointstructures 40/54 of the cephalad prosthesis 36 and caudal prosthesis 38can create an artificial articular configuration that is unlike thepre-existing natural articular configuration in terms of, e.g.,interpedicle distance, lardotic angle, and contact stress, so that adesired articulation or bony anatomy can be totally restored. At thesame time a desired bone anatomy is restored, decompression of theadjacent nerve root can be maintained to eliminate pain.

Further details of surgical procedures suitable for installing theprostheses are described in co-pending U.S. patent application Ser. No.09/693,272, filed Oct. 20, 2000, and entitled “Facet ArthroplastyDevices and Methods,” which is incorporated herein by reference.

The above described embodiments of this invention are merely descriptiveof its principles and are not to be limited. The scope of this inventioninstead shall be determined from the scope of the following claims,including their equivalents.

1-55. (canceled)
 56. An implantable device for facet joint replacementcomprising: a component sized to be fixed to a first vertebral body andincluding a joint structure adapted to replace a portion of a naturalfacet joint and articulate with a second vertebral body.
 57. Animplantable device according to claim 56 further comprising a fixationelement sized to accommodate adjustment of the component relative to thefirst vertebral body.
 58. An implantable device according to claim 56wherein the joint structure comprises a generally convex articulatingsurface.
 59. An implantable device according to claim 56 wherein thejoint structure comprises a generally concave articulating surface. 60.An implantable device according to claim 56 wherein the joint structurecomprises a biocompatible polymer.
 61. An implantable device accordingto claim 56 wherein the joint structure comprises a surface adapted toarticulate with a facet structure of a second vertebral body.
 62. Animplantable device according to claim 56 wherein the joint structurecomprises a surface adapted to articulate with an artificial structureon a second vertebral body.
 63. An implantable device according to claim56 wherein the joint structure is adapted to replace a cephalad portionof the facet joint.
 64. An implantable device according to claim 56wherein the joint structure is adapted to replace a caudal portion ofthe facet joint.
 65. An implantable device according to claim 56 whereinthe joint structure is adapted to replace a caudal portion and acephalad portion of the facet joint.
 66. An implantable device accordingto claim 56 further comprising a fixation element, wherein a fixationregion on the fixation element is configured and arranged to support thefixation element at a desired entry angle.
 67. An implantable deviceaccording to claim 56 further comprising a fixation element, the devicefurther comprising a fixation region on the fixation element that isconfigured and arranged to support the fixation element at an angle thatapproximates a desired lordotic angle.
 68. An implantable deviceaccording to claim 56 further comprising a first fixation regionconfigured and arranged to fix the component to the first vertebral bodyat or near a pedicle.
 69. An implantable device according to claim 56further comprising a first fixation region configured and arranged tofix the component to the first vertebral body at or near a lamina. 70.An implantable device according to claim 56 further comprising a firstfixation region configured and arranged to fix the component to thefirst vertebral body at or near a spinous process.
 71. An implantabledevice according to claim 56 further comprising a first fixation regionconfigured and arranged to fix the component to the first vertebral bodyat or near a spinous process.
 72. A method of replacing, on a vertebralbody, a portion of a natural facet joint using the implantable device asdefined in claim 56, the method comprising the steps of (i) removing atleast a portion of the natural facet joint from the vertebral body, and(ii) fixing the implantable device as defined in claim 1 to thevertebral body to replade the removed portion of the natural facet jointwith the joint structure.
 73. A device for replacing a portion of afacet joint between adjacent vertebral bodies, the device comprising: acomponent sized to be fixed to a first vertebral body and including astructure adapted to replace the portion of a facet joint, therebyallowing the first vertebral body to articulate with a second vertebralbody.
 74. An implantable device according to claim 73 further comprisinga fixation element sized to accommodate adjustment of the componentrelative to the first vertebral body.
 75. An implantable deviceaccording to claim 73 wherein the structure comprises a biocompatiblepolymer.
 76. An implantable device according to claim 73 wherein thestructure comprises a surface adapted to articulate with a facetstructure of the second vertebral body.
 77. An implantable deviceaccording to claim 73 wherein the structure comprises a surface adaptedto articulate with an artificial structure attached to a secondvertebral body.
 78. An implantable device according to claim 73 whereinthe structure is adapted to replace a cephalad portion of the facetjoint.
 79. An implantable device according to claim 73 wherein thestructure is adapted to replace a caudal portion of the facet joint. 80.An implantable device according to claim 73 wherein the structure isadapted to replace a caudal portion and a cephalad portion of the facetjoint.
 81. An implantable device according to claim 73 furthercomprising a fixation element, wherein a fixation region on the fixationelement is configured and arranged to support the fixation element at adesired entry angle.
 82. An implantable device according to claim 73further comprising a first fixation region configured and arranged tofix the component to the first vertebral body at or near a pedicle. 83.An implantable device according to claim 73 further comprising a firstfixation region configured and arranged to fix the component to thefirst vertebral body at or near a lamina.
 84. An implantable deviceaccording to claim 73 further comprising a first fixation regionconfigured and arranged to fix the component to the first vertebral bodyat or near a spinous process.